Iron loss which is a very important factor for a core used in electric transformers is defined by eddy-current loss which is affected by a resistivity of the core, and hysteresis loss which is affected by strain in a soft magnetic powder, which is generated in a production process of the soft magnetic powder and subsequent processing steps. The iron loss W can be specifically defined by a sum of eddy-current loss We and hysteresis loss Wh as shown in the following formula (1). In the formula (1), expression in front of the plus sign is the eddy-current loss We and the expression after the plus sign is the hysteresis loss Wh. In the formula, “f” is frequency, “Bm” is exciting magnetic flux density, “ρ” is resistivity, “t” is material thickness, and “k1” and “k2” are coefficients.W=We+Wh=(k1Bm2t2/ρ)f2+k2Bm1.6f  (1)
As shown in the formula (1), while the hysteresis loss Wh is proportional to the frequency f, the eddy-current loss We is proportional to the square of the frequency f. Therefore, in order to decrease the iron loss W, specifically in a high frequency area, it is effective to decrease the eddy-current loss We. In order to decrease the eddy-current loss We, it is necessary to increase the resistivity ρ by limiting the eddy-current in a small area. In a powdered core obtained by using powders, for example, nonmagnetic resin can exist between iron powder particles, and the like. Therefore, the powdered core has essential characteristics in which the resistivity ρ is high and the eddy-current loss We is small. Conventionally, a production technique for a powdered core was proposed in Japanese Laid-open Patent Application No. S60-235412 (pages 1 and 2) in which a mixture of a soft magnetic powder and a resin powder was used, and compacting and heating were performed. In the powdered core described in the above laid-open application, resin existed between soft magnetic powder particles. Therefore, electrical insulation between the soft magnetic powder particles was specifically assured, whereby the eddy-current loss We was decreased, and the soft magnetic powders were tightly bound, whereby strength of the powdered core was increased.
The powdered core mentioned above has been widely used because it is easy to produce. However, when the powdered core is used in a high frequency area, the insulation characteristics are not sufficient, whereby the resistivity ρ is decreased, resulting in increasing the eddy-current loss We. The increase in eddy-current loss We causes heat generation, whereby resin binding the soft magnetic powder is deteriorated. Therefore, the powdered core has a disadvantage in that sufficient durability cannot be obtained. On the other hand, when the resin amount is increased in order to increase the electrical insulation, the amount of the soft magnetic powder contained in the core (packing factor) is decreased, whereby the magnetic flux density is decreased. Therefore, it is important to increase the magnetic flux density by increasing the density of the powdered core. However, in this case, the powder must be compressed at a high pressure, and strain is generated in the soft magnetic powder in compacting. Therefore, the hysteresis loss Wh would increase and the iron loss W would increase. Specifically in a low frequency area, the eddy-current loss We is small, whereby effect of the hysteresis loss Wh for the iron loss W is large. Decrease in the hysteresis loss Wh is also important in order to decrease the iron loss W.
The powdered core is used for electromagnetic actuators such as solenoids and motors. High attraction power and high responsiveness are required in an electromagnetic valve used for a fuel injector in a diesel engine, and high magnetic flux density and small eddy-current loss We in a high frequency area are preferable in stator core materials using the powdered core. These solenoid cores are powdered cores which are obtained by compacting a mixture of iron powder and a resin powder. High density and favorable electrical insulation between iron powder particles are required in the solenoid cores so as to increase the magnetic flux density and to decrease the iron loss.
In various motors, small size and high efficiency are required and high magnetic flux density and small eddy-current loss We in a high frequency area are also preferable in a rotor and stator material using a powdered core. Required characteristics for a powdered core used in various electromagnetic actuators are essentially the same as those for a core used in an electric transformer.
In order to obtain a powdered core with high magnetic flux density, high density of the powdered core is necessary, and the compacting pressure must be not less than two times the pressure in producing ordinary sintered alloys. In a powdered core with a complicated shape or a thin wall, the durability of a compacting die assembly would be deteriorated. In a powdered core having a shape similar to a solenoid core, the powdered core compacted to a simple cylindrical or columnar shape is machine finished into a predetermined shape and dimension. Alternately, a powdered core compacted to a shape close to a product shape is machine finished at portions in which dimension accuracy is specifically required. Therefore, the powdered core is required to have excellent machinability, whereby wear of cutting tool can be small and breakage and chipping of the material in machining can be prevented.
A magnetic flux density of the powdered core depends on material density thereof, whereby atomized iron powder in which relatively high density can be obtained is used as an iron power. In a surface of the iron powder, a phosphate compound is coated in order to decrease an iron loss of the powdered core. As resin powders mixed with the iron powder, it is proposed to use phenol, polyamide, epoxy, polyimide or polyphenylene sulfide. For example, Japanese Laid-open Patent No. 2002-246219 (summary) discloses a powdered core obtained by adding a resin selected from polyphenylene sulfide, thermosetting polyimide, and the like at 0.15 to 1 mass % to atomized iron powders coated with phosphate compound. Japanese Patent Publication No. 421944 (section 36) discloses a powdered core obtained by adding a thermosetting polyimide resin at 2 mass % to atomized iron powders coated with phosphate compound.
According to these findings, in order to simultaneously decrease the eddy-current loss We and the hysteresis loss Wh, various methods for decreasing the eddy-current loss We by assuring electrical insulation between the soft magnetic powder particles by preliminarily coating insulation film over the surface of the soft magnetic powders are disclosed, for example, Japanese Laid-open Patent No. H9-102409 (pages 6 and 7). In a technique described in the above laid-open disclosure, a process in which the insulation film is coated on the surface of the soft magnetic powder is essential, thereby having a disadvantage of having high production cost. Recently, it has been requested to develop the production method for a powdered core in which low production cost can be realized, the eddy-current loss We and the hysteresis loss Wh are decreased, and durability of the powdered core can be improved.
In the solenoid core made of the above-mentioned powdered core, higher magnetic flux density and smaller iron loss are required. Furthermore, when the solenoid core is machined (including drilling) for shaping and assuring the dimension accuracy thereof, it is required to have enough strength to withstand chucking and without breakage, peeling, and chipping in the machining process.
The present invention has been made to essentially realize low production cost without performing special processes including coating of insulation film. A object of the present invention is to provide a production method for a powdered core, in which electrical insulation is increased by uniformly disposing a resin between soft magnetic powder particles, whereby the eddy-current loss We in a high frequency area and heat generation caused by the We are decreased, thereby improving the durability of the powdered core and improving performance of products using the powdered core. Another object of the present invention is to provide a production method for a powdered core, in which magnetic flux density is sufficiently assured by thinly disposing the resin between the soft magnetic powder particles, whereby the hysteresis loss Wh and heat generation caused by the Wh are decreased, thereby further improving the durability of the powdered core and improving performance of products using the powdered core. In the case of coating the insulation film over the surface of the soft magnetic powder in the present invention, there is an additional object that the electrical insulation is assured at higher levels and the magnetic flux density is further increased by decreasing the resin amount used, whereby the durability of the powdered core is further improved.